This invention relates to highly efficient ballast circuit especially suitable for use with fluorescent lamp loads and more particularly to tuned oscillator ballast circuitry employing feedback from the load circuitry for driving the oscillator circuitry.
Presently, auto-transformer type ballasts are commonly employed for fluorescent lamp systems. Such apparatus is known to be undesirably heavy and cumbersome as compared with electronic forms of ballasts. Also, it is known that such apparatus is relatively inefficient, generates undesired heat, is undesirably wasteful of energy, and operates at a frequency (60Hz) which is undesirably in the audible range.
Another popular form of ballast circuitry employs a flip-flop oscillator and a saturable core transformer. Core saturation characteristics are utilized to limit current flow which is difficult to control and accurately predict. Thus, such apparatus tends to lack the reliability desired in such equipment.
Another form of ballast circuitry, set forth in a co-pending application assigned to the assignee of the present invention, provides a sinusoidal oscillator circuit which includes circuitry to compensate for "storage time" of the transistors of the oscillator circuit. Even though such circuitry has enhanced capabilities, as compared with prior known apparatus, it has been found that improved efficiency is possible without undue increase in cost.
A further form of ballast circuit, set forth in a copending application assigned to the assignee of the present application provides a storage capability whereby the pulsed DC potential applied to a tuned oscillator is altered to provide application of a substantially constant DC potential to a lamp load. However, it has been found that such circuitry leaves something to be desired with respect to sudden changes in loading. Thus, added protection capabilities would be highly desirable.